*anonymous user* , Welcome to Microsoft Q&A! Thanks for posting a good question.
Azure WebApps by default have ARR Affinity cookie enabled, this cookie pairs a client request to a specific server. However, Azure Web Apps is a stateless platform and, in an environment, where we are scaling the Website across multiple instances, the ARR Affinity cookie will be bound to a specific server. In case, the server is no longer in rotation, then all the requests corresponding to the ARR Affinity cookie will fail. It’s advisable to avoid the use ARR cookies in a scaled environment where we have multiple instances that serve our application requests. Disabling ARR cookies is a sustainable resolution for issues related to ARR Affinity cookies in scaled environments, where these cookies rely on the relationship with the worker machine they are paired with.
Also, when we have ARR Affinity cookies enabled, we are truly not utilizing the scaling ability where a requests can get handled by any instance that has the resources to handle the request. Instead, based on ARR Affinity cookie mapping, the requests will always go to only to the server tied to the Affinity cookie.
If your application is stateful, scaling up would be best, while if your application is stateless, scaling out gives you more flexibility and higher scale potential.
For more information about “stateful” vs “stateless” applications kindly watch the following video:
Planning a Scalable End-to-End Multi-Tier Application on Azure App Service.